Trapezoidal belt variable-speed drives are commonly used on small vehicles such as snowmobiles, scooters or small cars. Such drives mainly comprise a driving pulley, a trapezoidal belt and a driven pulley. The driving pulley is linked to an engine and the driven pulley is usually mechanically connected to ground traction means, such as wheels or tracks.
The main object of using a variable-speed drive is to automatically change the winding diameter of the trapezoidal belt around the driving and the driven pulleys in order to have a maximum torque at low speeds and a reasonable engine rotation speed at high speeds. The sides of the trapezoidal belt are, on each pulley, gripped between two opposite flanges wherein one is fixed and one is axially movable. At low speeds, the winding diameter of the driving pulley is small and the winding diameter of the driven pulley is maximum. As the rotation speed of the driving pulley increases, the movable flange of the driving pulley gets closer to the fixed flange and thus forces the trapezoidal belt to wind on a greater diameter. Since the trapezoidal belt is not substantially stretchable, the trapezoidal belt exerts a radial force towards the center on the flanges of the driven pulley in addition to the tangential driving force to compensate for the increasing winding diameter of the driving pulley. This radial force constrains the driven pulley to have a smaller winding diameter. Therefore, the movable flange of the driven pulley moves away from the fixed flange until the return force exerted by a spring counterbalances the radial force exerted by the trapezoidal belt. It should be noted at this point that a change in the load also produces a variation of the winding diameter of the pulleys, a greater load inducing a greater winding diameter of the driven pulley.
When the rotation speed of the engine decreases, the winding diameter of the driving pulley decreases and the radial force exerted by the trapezoidal belt decreases, thus allowing the driven pulley to have a greater winding diameter.
In conventional driven pulleys, the movable flange is provided with a plurality of slider buttons at the back thereof. These buttons have inclined surfaces that are adapted to be set against a cam plate solid with the shaft. The cam plate has inclined surfaces at equal distance around the shaft. The inclined surfaces allow the movable flange to move away or towards the fixed flange while still keeping a rotational engagement with the cam plate, therefore with the shaft. In such driven pulleys, the fixed flange is solid with the shaft. Then, as a result, there is a relative rotation of the movable flange with reference to the fixed flange as it moves away or towards it. One of the drawbacks of such relative movement between flanges is that one side of the belt has to "slip" on the movable flange until a new equilibrium is reached in the driven pulley while the other side of the belt remains substantially in full contact. Wear is thus likely to happen on one side of the belt. Additionally, the slip increases the response time and reduces the maximum torque that can be transferred through the driven pulley.
Another drawback of conventional driven pulleys is that the belt used in a conventional drive is also used as a clutch lining for positively engaging the drive into action when accelerating from full stop or from a very low speed. This is done by the driving pulley where the flanges are not initially engaged with the belt and where an engagement occurs as the movable flange moves towards the fixed flange due to an increase in the motor speed. The condition of the belt is likely to deteriorate after a number of starts because of the high friction, especially if the load is very high or if the start is very sudden. Although the materials used for making belts are more resistant than in the past, the belts are still not suitable as clutch linings.